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CELLS AND CELL BIOLOGY
Define the term homeostasis and explain why it is vital for normal body function. Homeostasis is the maintenance of the conditions within the cell or within the body that maintain life, despite changes that may be occurring in or outside the body. Homeostasis is essential to many physiological processes which occur in the body. For example, the metabolic functions that occur in the body require a temperature of 370C. If this temperature were to change, metabolic processes would be affected. Most disease states can be regarded as a result of disturbance of homeostasis, a condition called homeostatic imbalance. Define and explain the terms negative feedback and positive feedback as they apply to homeostatic control and give examples of each. Negative feedback-causes the variable to change in a manner that is opposite to the initial change returning it to its normal value. Example-when body temperature rises (or falls) receptors in the skin and hypothalamus sense a change and coordinate the appropriate response which would be a decrease (or increase) in body temperature. Positive feedback'-'''the change that results proceeds in the '''same direction '''as the initial change, causing the variable to deviate further from its original value/range. Example-oxytocin, a hypothalamic hormone, intensifies labour contractions during the birth of a baby. Oxytocin causes the contractions to become both more frequent and more powerful. The increased contractions cause more oxytocin to be released, which causes more contractions and so on until the baby is finally born. The birth ends the stimulus for oxytocin release and shuts off the positive feedback mechanism. '''Define pH and its significance in the human body' pH is defined as the negative decimal logarithm of the hydrogen ion concentration of a solution in moles per litre. 7 – neutral, >7- alkaline, <7 – acidic Ph balance is important in maintaining homeostatic imbalance in the body. The Ph of arterial blood plasma is 7.40. If it falls below this value, a condition called acidosis is produced, if it rises above this value, alkalosis results. Biological buffers help to prevent excessive changes in the ph of body fluids. Explain the importance of water in the human body. Water is the most important inorganic molecule and makes up approximately 80% of our body value. It is an essential nutrient required by the body. What makes water so vital? The answer lies in several properties: High heat capacity-water absorbs and releases large amounts of heat before changing appreciably in temperature itself, it prevents sudden changes in temperature. As part of blood, water redistributes heat among body tissues ensuring temperature homeostasis. High heat of vaporization-as perspiration (mostly water) evaporates from our skin, large amounts of heat are removed from the body providing efficient cooling. Polar solvent properties-biological molecules do not react unless they are in solution and virtually all chemical reactions occurring in the body depend on water’s solvent properties. Reactivity-water is an important reactant in many chemical reactions.For example, foods are digested to their building blocks by adding a water molecule to each bond to be broken. For example, foods are digested to their building blocks by adding a water molecule to each bond to be broken. Cushioning-water cushions body organs helping to protect them from physical trauma. Example- the cerebrospinal fluid that surrounds the brain. Distinguish between cofactors and coenzymes and their role in human physiology Enzyme-A protein catalyst that increases the reaction rate without being affected in the overall process. The majority of biochemical reactions that occur in the body are carried out by enzymes. Some enzymes are purely protein. In other cases, the functional enzyme consists of 2 parts, collectively called a holoenzyme: an apoenzyme (the protein portion) and a cofactor. Some enzymes require help from other molecules in order to carry out its reaction. This molecule is called a cofactor. The cofactor may be an ion of a metal element (zinc, iron) or an organic molecule. Most organic cofactors are derived from vitamins (especially the B vitamins) and are called coenzymes. Role in human physiology Coenzymes support the functions of enzymes- they act as transporters of chemical groups from one reactant to another. Examples: NAD- nicotinamide adenine dinucleotide is a coenzyme formed from vitamin B3 and is involved in several metabolic processes. Coenzyme Q-involved in the production of energy within the body’s cells. ATP is also regarded as a common coenzyme. Biological functions of enzymes within the human body Enzymes are biological catalysts responsible for supporting almost all of the chemical reactions that maintain homeostasis and are found in all tissues and fluids of the body. Some examples: Digestion-amylases and proteases Signal transduction and cell regulation-kinases and phosphatases Generation of movement-myosin hydrolyses ATP to generate muscle contraction Regulation of blood clotting- prothrombin activator Diagnosis-eg. Elevated CPK levels may indicate acute infarction Describe the transportation of of substances across the cell membrane. Explain the difference between diffusion, osmosis and active transport and their role in maintaining cellular structure and function. The plasma membrane acts as a selectively permeable barrier. Substances move across the membrane by: Passive processes- depend on kinetic energy Active processes-depend on cellular enegy (ATP) Diffusion-'''movement of molecules (driven by kinetic energy) down a concentration gradient. Example- diffusion allows oxygen to diffuse from the blood into cells and carbon dioxide to diffuse from cells into the blood. '''Osmosis'-'''diffusion of a solvent (such as water) through a plasma membrane. It occurs when the water concentration differs on the 2 sides of the membrane. Osmosis is important in determining the distribution of water in the various fluid-containing compartment of the body eg. Cells, blood. '''Active transport-'''movement of molecules against their concentration gradient requiring energy in the form of ATP to facilitate the process. Active transport requires proteins termed “pumps” to move the molecules against their concentration gradient. '''Primary active transport-'''the energy to do work comes '''directly from the hydrolysis of ATP. ' Primary active transport systems include the sodium-potassium pump which maintains the resting membrane potential. Secondary active transport-'''the transport is driven '''indirectly by energy stored in ionic gradients created by the operation of primary active transport pumps. It is based on the electrochemical potential difference that is created by pumping ions out of the cell. The electrochemical gradients maintained by the sodium-potassium pump underlie most primary and secondary active transport of ions and nutrients, and are crucial for cardiac and skeletal muscle and neuron function. Define membrane potential and explain how the resting membrane potential is established and maintained. Membrane potential-voltage difference across the plasma membrane that occurs due to separation of ions. Resting membrane potential-voltage that exists across the plasma membrane during the resting state of an excitable cell and ranges from -50 to -100 millivolts depending on the cell type. How is the resting membrane potential established? The resting membrane potential is established by the concentration gradient of ions and the differential permeability of the plasma membrane to ions, particularly potassium ions. Potassium leaves the cell along the concentration gradient. As more potassium leaves the cell, the negativity of the inner membrane attracts potassium back into the cell. At this point a negative membrane potential (-90Mv) is established when movement of potassium out of the cell equals potassium movement into the cell. Sodium also contributes to the resting membrane potential. Sodium is strongly attracted into the cell by its concentration gradient bringing the resting potential to -70 millivolts. Potassium largely determines the resting potential since the plasma membrane is more permeable to potassium than to sodium. How is it maintained? The resting membrane potential is maintained by the sodium-potassium pump by preventing an electrochemical equilibrium across the membrane, caused by the passive ion movement “leaky ion channels”. The sodium-potassium pump couples sodium and potassium transport: each turn of the pump ejects 3 sodium ions out of the cell and carries 2 potassium ions back in. ·